The damage induced by low LET ionizing radiation elicits a complex series of responses from the cell that determine the final outcome. Potential cellular responses are both epigenetic and genetic, including repair of the damage, arrest of proliferation, alteration of gene expression, and cell death. Genotoxic agents elicit a wide variety of final outcomes ranging from embryonic failure to tumorigenesis; however, few studies have addressed the nature of responses to low doses of such agents. This project will analyze the cellular and molecular mechanisms underlying the tissue and stage-specific variation in cellular response to low exposures of X-irradiation (0.05-0.5 Gy). These studies will exploit the observation that early mammalian embryos show a high sensitivity to genotoxic agents with outcomes of embryo failure and malformations, to develop an assay system in which normal and transgenic mice will be used to elucidate the cellular and molecular processes underlying these processes in studies in culture and in vivo. An in vitro mouse embryo culture system for the peri-implantation stages of development (equivalent to 4-7 days of development in vivo) will be established in which the response of cells to X-irradiation will be determined by following their developmental fate, proliferation and cell death. Regulation of the cell cycle appears to play a central role in coordinating the decisions about proliferation, apoptosis and DNA repair in response to DNA damage. The expression and activity of the molecules involved in cell cycle progression and checkpoints will be examined. Transgenic mice with mutant or inactive forms of regulatory molecules (p53, RB or cd2) will be used to evaluate the role of these molecules in the response to irradiation directly. Determining the molecular basis of the response to low dose X-irradiation will help define classes of environmental compounds that are likely to have biological effects. These studies will help elucidate the basis of individual and tissue-specific sensitivity, particularly during development, and will evaluate a potential assay sensitive to biologically relevant doses of such agents, in which it is possible to evaluate their potency and mode of action.